1. Field of the Invention
This invention relates to a digital signal processing device for switching and outputting a 1-bit digital signal obtained on sigma-delta modulation, a recording device for recording the 1-bit signal and a reproducing device for reproducing the 1-bit digital signal.
2. Description of the Related Art
The high-speed 1-bit signal, obtained on sigma-delta (.SIGMA..DELTA.) modulation, has an extremely high sampling frequency and a short word length, such as a sampling frequency equal to 64 times 44.1 kHz and a word length of 1 bit, as compared to a so-called multi-bit digital signal with the sampling frequency of 44.1 kHz and a data word length of 16 bits, used in the conventional digital audio, and is characterized by a broad transmission frequency range. In addition, the high-speed 1-bit signal has a dynamic range sufficiently higher than the conventional audio range of 20 kHz with the sampling frequency of 44.1 kHz.times.64=2.8224 MHz. By exploiting these properties, the 1-bit signal can be used for a recorder with a high sound quality or data transmission.
The circuit employing the .SIGMA..DELTA. modulation itself is not a new technique and has so far been used every so often in an A/D converter because the circuit structure can be easily built into an IC and high precision can be realized relatively easily. The .SIGMA..DELTA. modulated signal can be passed through a simple analog low-pass filter for re-conversion to analog audio signals.
Meanwhile, the above-described high-quality 1-bit audio signals tend to be increased in data capacity as compared to the conventional multi-bit system. For saving the data capacity, a 1-bit audio signal with a lower sampling frequency may be used. However, in this case, the signal quality is inevitably lowered. It is therefore contemplated to switch between a 1-bit audio signal with a high sampling frequency and a 1-bit audio signal with a low sampling frequency in the same device from one media to another and from one quality that is optionally required in a given media to another, and to record the selected 1-bit audio signal.
Meanwhile, since the 1-bit audio signal is a signal modulated on the time axis, if the 1-bit digital signals of two different routes and with different sampling frequencies are switched directly, a large noise is produced at the switching point.